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Malware housed on storage and caching servers, such as those used by ISPs, enterprises, and leading search engines, continues to pose a risk after websites containing malicious code have been pulled.

So says web security firm Finjan, which warns that instead of pointing users towards sites hosting malware, hackers could try to dupe users into visiting contaminated caches. The trick might be used to foil URL filtering products, it says.

"This is more than just a theoretical danger," Finjan chief technology officer Yuval Ben-Itzhak said. "It is possible that storage and caching servers could unintentionally become the largest 'legitimate' storage venue for malicious code. Such 'infection-by-proxy' introduces new risks for businesses and consumers where trusted web addresses become a potential distributor of malicious code - making URL Filtering solutions blind."

Finjan has published obfuscated examples of malware found on storage and caching servers to support its claims.

One well-known hacking tactic involved breaking into vulnerable web servers to install Trojan downloader code, which often takes advantage of browser vulnerabilities to download malware onto target PC (examples here and here). Finjan's point is that users visiting a cached copy of such (potentially mainstream) sites would be infected even if the main site pulled the malware. Search engines are not doing enough to flush their caches, it warns."

Finjan has sent search engines and service providers technical details of its discovery, uncovered by Finjan's Malicious Code Research Centre (MCRC) during its quarterly security trends analysis, and is continuing its dialogue with these firms in the hope of nipping the problem in the bud.

Finjan's net security report, which also discusses the increased use by hackers of Web 2.0 technologies to upload malware and the illicit trade in exploit code, can be found here (registration required). ®